Unknown Hissing

[rquaale]

When ever my truck is idleing I hear a hissing sound under the hood, but it doesnt throw any codes or have any driveability issues. The a/c and everything else blows out of where it should. What would cause the hissing but not cause any noticeabe issues? I noticed the sound when I was refilling my a/c system and according to the instructions on the can the vehicle needed to be running and the a/c system needed to be turned on in order for it to work.

 

[RonD]

A hissing would be a vacuum most likely, louder at idle and less noticeable as RPMs are increased/vacuum decreases.

Fuel injected engines use a "controlled vacuum leak" to set the idle level.
This is called the IAC(idle air control) valve, it is usually located on the upper intake, looks like a cylinder and can be removed and cleaned.
The IAC pulls in air to set the idle, so can produce a hissing noise, but usually not to noticeable over engine noise, it's air intake tube could be dirty.
This wouldn't effect performance unless computer couldn't set a stable idle by opening and closing the IAC valve.

You can use a short piece of hose as a stethoscope to see exactly where the hiss is coming from.


3.0l engines can start to get a high pitch squeal, that's the Cam Position sensor/synchronize, this would be a concern.
Read here:
http://www.therangerstation.com/tech_library/camshaft_position_sensor.html